1. Field of the Invention
The present invention relates to a pitch changing device for changing intervals between adjacent plate-like objects arranged in a container and a method of changing pitches and, more particularly, to a pitch changing device for changing intervals of a large number of semiconductor wafers when the wafers are transferred between a cassette and a boat, and a method of changing pitches.
2. Description of the Related Art
In a conventional semiconductor element manufacturing device, 25 sheets of semiconductor wafers are loaded on a quartz-boat. Then, the boat is placed in an annealing furnace, and, e.g. 150 sheets of wafers are simultaneously annealed. A transfer device is disposed in front of a port of the annealing furnace. The semiconductor wafers in the cassette are sequentially transferred to the boat by the transfer device.
The transfer device has a pair of clamp members (chuck members) for clamping the wafers. A large number of grooves are formed in a wafer clamp surface of each clamp member at equal intervals (grooves pitch of the cassette or the boat). An edge of each wafer is inserted in the corresponding groove, so that the wafers are held.
The grooves of the clamp members are normally formed at the same pitches as those of the grooves in the inner surface of the cassette, i.e., at pitches of 3/16 inches. In general, a maximum of 25 semiconductor wafers can be contained in one cassette. On the contrary, one boat can generally load a maximum of 200 wafers per lot. In this case, since the annealing conditions are changed for each charge, pitches of the wafers in the boat, i.e., pitches of 3/16 inches are often required to be different from those in the cassette. For example, the wafers are arranged on the boat at pitches of 6/16 inches, 9/16 inches, 1/8 inches, or the like.
Depending on a user, "millimeter" is often designated as a unit of pitch of the wafer arrangement, in place of "inch".
Thus, when the pitches are changed depending on the boat or cassette, since grooves having unchangeable pitches are formed in the clamp members of the conventional transfer device, the pitch change cannot be performed.
According to Japanese Utility Model Disclosure (Kokai) No. 6166944, as shown in FIG. 1, a chuck mechanism 2 wherein intervals between a plurality of chucks 2a are variable is disclosed as clamp members of a transfer device. More specifically, the conventional pitch changing device is constituted by the chuck mechanism 2 including the plurality of chucks 2a for holding a plurality of wafers 1 so that surfaces of the wafers are parallel to each other, a plurality of link members 3 for independently driving chucks 2a to variably change pitches between the adjacent wafers 1, and driving mechanisms 4 for driving these link members 3 at the same time. Fulcrum 3a of the link members 3 are different from each other.
In this device, when the driving mechanisms 4 are rotated, the first ends of the link members 3 are moved by equal distances, and chuck members 2a mounted at the second ends of the link members 3 are moved by different distances depending on their fulcrum positions. As a result, pitches between the adjacent chuck members 2a are changed.
In the above-mentioned conventional device, however, each link member 3 is required for the corresponding chuck member 2a. In addition, since the fulcrums 3a of the link members 3 are different from each other, the device is undesirably complicated, and has a large size.
In addition, in the above-mentioned device, strictly speaking, when the first ends of the link members 3 are moved by equal distances, orbits of the second ends of the link members 3 have an arc-shape. For this reason, although the chuck members 2a coupled to the second ends are guided to linearly slide by a guide, the chuck members 2a tend to be moved along the arcs. Therefore, a moving resistance is large, and smooth movement cannot be realized.
A wafer transfer device including clamp members for allowing a variable change in pitches is disclosed in Japanese Patent Disclosure (Kokai) Nos. 61-244040 and 54-161881. However, since the structures of the devices are complicated and easy handling cannot be achieved. Therefore, both the devices are not practically used.
In the above-mentioned conventional device, when pitches of the wafers are simultaneously changed while each wafer is clamped by the corresponding pair of clamp members, all the pitch changing operations are not completely synchronized. Therefore, an excessive force acts on the wafers, so that the wafers are often damaged.